Cohen was hired in June 2017 to oversee the firm's EMEA consumer work, which made up more than a quarter of its business in the region at the time. He reported to EMEA EVP Andres Wittermann.
Giles Peddy, the firm's UK MD and SVP EMEA operations, said: "We can confirm that Daniel Cohen has decided to leave agency for new challenges. He has made a great contribution in his time with us, and we wish him all the best for the future."
Lewis is not recruiting for a like-for-like replacement for Cohen.
Cohen arrived at Lewis following a 13-year stint at Grayling, its consumer offshoot Atomic and forerunners of those firms, latterly as global head of brands.
He told PRWeek he was yet to decide on his next step.
Grayling announced the hire of Andrew Ferguson as creative director shortly after Cohen's departure.